Spectroscopic observations of exoplanets are leading to unprecedented constraints on their atmospheric compositions .
However , molecular abundances derived from spectra are degenerate with the absorption cross sections which form critical input data in atmospheric models .
Therefore , it is important to quantify the uncertainties in molecular cross sections to reliably estimate the uncertainties in derived molecular abundances .
However , converting line lists into cross sections via line broadening involves a series of prescriptions for which the uncertainties are not well understood .
We investigate and quantify the effects of various factors involved in line broadening in exoplanetary atmospheres - the profile evaluation width , pressure versus thermal broadening , broadening agent , spectral resolution , and completeness of broadening parameters - on molecular absorption cross sections .
We use H _ { 2 } O as a case study as it has the most complete absorption line data .
For low resolution spectra ( R \lesssim 100 ) for representative temperatures and pressures ( T \sim 500K-3000K , P \lesssim 1 atm ) of H _ { 2 } -rich exoplanetary atmospheres we find the median difference in cross sections ( \delta ) introduced by various aspects of pressure broadening to be \lesssim 1 % .
For medium resolutions ( R \lesssim 5000 ) , including those attainable with JWST , we find that \delta can be up to 40 % .
For high resolutions ( R \sim 10 ^ { 5 } ) \delta can be \gtrsim 100 % , reaching \gtrsim 1000 % for low temperatures ( T \lesssim 500K ) and high pressures ( P \gtrsim 1 atm ) .
The effect is higher still for self broadening .
We generate a homogeneous database of absorption cross sections of molecules of relevance to exoplanetary atmospheres for which high temperature line lists are available , particularly H _ { 2 } O , CO , CH _ { 4 } , CO _ { 2 } , HCN , and NH _ { 3 } .